When I was much younger I used to earn 6 figures, and whilst I never considered myself wealthy as such, I never worried about money, ever. Ate out all the time, thought nothing of going into London and dropping £500 on a new outfit or bag, did my supermarket shop without even looking at prices, gave no thought at all the things like shopping around for energy tariffs. Saved between £1,000 - £2,000 a month. If I'd wanted a Porsche I could have afforded to lease one no problem.
I'm now on a very average income, new clothes are a treat and always from the high Street or ebay, can't remember the last time I went out to eat. Whilst I still spend more than I should at the supermarket (working on that!) I do shop around and look for discounts, meal plan etc. My car is 9 years old, but it's paid for. Some months I manage to save £100-200ish. Someone up thread mentioned pay day not being notable - it is very much in our house because as we get close to it the bank account starts to look pretty empty and it's nice to have it topped up again!
Even in comparison to my 'old' life I would still describe myself as comfortable - I can afford to feed myself with more than the basics, whilst I'm careful about energy usage I can put the heating on without panicking. I have enough good quality warm clothes and shoes. Our house is nicely decorated, cosy and warm, although i have a million things I'd do to it/ buy for it if I had more money. If my car broke down I'd have to put any significant costs on a credit card, but I'd be able to pay it off over a few months.
Not being comfortable to me means not being able to afford more than the very basics, and not being able to comfortably fund in some way an emergency purchase such as car repairs or a new washing machine etc.